The research of cardiovascular functions of the area postrema (AP) in rats is extended into ongoing investigations of a role for AP noradrenergic (NA) neurons in the modulation of baroreflex in rabbits. A localized and selective lesion of AP-NA projections to the baroreceptive neurons in the medial nucleus of the solitary tract (mNTS) will be utilized in neuroanatomical and physiological experiments designed to test the hypothesis that NA neurons within the rabbit AP modulate the baroreceptor reflex through converging axodendritic facilitatory mechanisms within the mNTS. By first establishing the anatomical existence of these synapses, the altered responsiveness to chronic infusion of vasopressin will be correlated with anatomical relationships of aortic baroreceptor afferents in the mNTS. Blood pressure, heart rate, and sympathetic nerve activity will be monitored. This research will increase our understanding of cardiovascular regulatory mechanisms involved in treating hypertension in human patients.